1. Field of the Invention
This invention relates to a noise erasing method for a stimulable phosphor sheet wherein, after a stimulable phosphor sheet which has a radiation image stored thereon is exposed to stimulating rays causing the stimulable phosphor sheet to emit light, the amount of light being proportional to the amount of energy stored thereon during exposure to radiation, in order to read out the radiation image, the stimulable phosphor sheet is exposed to erasing light thus releasing the energy remaining thereon.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
The applicant proposed in U.S. Pat. No. 4,651,220 and U.S. patent application Ser. No. 251,982 reveal systems for recording and reproducing an electron microscope image wherein a stimulable phosphor sheet is utilized to record and reproduce an electron microscope image with a high sensitivity and with good image quality, and wherein an electric signal which represents the electron microscope image is directly obtained so that various kinds of processing operations are facilitated. Basically, the proposed methods of recording and reproducing an electron microscope image wherein a stimulable phosphor sheet is utilized comprise the steps of (i) exposing a stimulable phosphor sheet to an electron beam which has passed through a sample in a vacuum in order to store the energy of the electron beam on the stimulable phosphor sheet, (ii) thereafter exposing the stimulable phosphor sheet to stimulating rays in order to release the stored energy in the form of emitted light, (iii) photoelectrically detecting the emitted light to obtain an image signal, and (iv) using the image signal to reproduce an electron beam image of the sample.
In the aforesaid systems for recording and reproducing a radiation image or an electron microscope image (the radiation image and the electron microscope image are herein generically referred to as a radiation image), all energy stored on the stimulable phosphor sheet during exposure to radiation would be released if the stimulable phosphor sheet were exposed to stimulating rays having a substantially high intensity when the radiation image is read out from the stimulable phosphor sheet. Actually, however, the intensity of stimulating rays irradiated to the stimulable phosphor sheet is not very high, and part of the energy stored on the stimulable phosphor sheet during exposure to radiation remains unreleased after the radiation image is read out from the stimulable phosphor sheet. Therefore, in cases where the stimulable phosphor sheet is used repeatedly to record and reproduce radiation images, the energy remaining on the stimulable phosphor sheet causes noise and results in an image signal detected from the stimulable phosphor sheet. In order to release the energy remaining on the stimulable phosphor sheet, the stimulable phosphor sheet may be exposed to erasing light after the radiation image is read out from the stimulable phosphor sheet, as disclosed in, for example, U.S. Pat. No. 4,400,619. If the amount of the erasing light irradiated to the stimulable phosphor sheet were to be set to a very large value, good noise erasing effects would be obtained. However, for the sake of economy, the amount of the erasing light should not be set to an unnecessarily large value. Accordingly, methods have been proposed in, for example, U.S. Pat. Nos. 4,584,482 and 4,687,937, wherein the level of energy remaining on a stimulable phosphor sheet is detected from information detected from the stimulable phosphor sheet, and the amount of erasing light irradiated to the stimulable phosphor sheet is adjusted approximately at the minimum necessary value in accordance with the detected level of energy remaining on the stimulable phosphor sheet.
In cases where a radiation image of a human body is recorded on a stimulable phosphor sheet and is read out therefrom, a very high radiation dose is not applied to the stimulable phosphor sheet because radiation is harmful to the human body. However, in cases where an electron microscope image of a sample is recorded, a very large amount of electron beams are often irradiated to a stimulable phosphor sheet. In such cases, light having a markedly high intensity is emitted by the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to stimulating rays in order to read out the electron microscope image from the stimulable phosphor sheet. The range over which image signals can be gathered is determined in advance with respect to the levels of image signals which are ordinarily obtained through photoelectric detection of light emitted by stimulable phosphor sheets. Therefore, in cases where an image signal has a markedly large value, the gathered signal value is clipped at the maximum value of the predetermined signal gathering range.
In such cases, the level of energy remaining on the stimulable phosphor sheet cannot be detected from information which has been read out from the stimulable phosphor sheet. Therefore, the amount of erasing light which is irradiated to the stimulable phosphor sheet cannot be set approximately to the minimum value necessary for the stimulable phosphor sheet.